Chemical Ecology: The Chemistry of Biotic Interaction docx

Many plant secondary compounds, for example, are inducible by UV light and presumably serve to protect or “defend” plants from damaging effects of UV exposure 21; by no stretch of the im

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The Chemistry of Biotic Interaction

Thomas Eisner and Jerrold Meinwald

Editors National Academy of Sciences

NATIONAL ACADEMY PRESS

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Chemical Ecology: The Chemisiry of Biotic Interaction (1995)

NATIONAL ACADEMY PRESS - 2101 Constitution Avenue, N.W - Washington, D.C 20418

This volume is based on the National Academy of Sciences’ colloquium entitled

“Chemical Ecology: The Chemistry of Biotic Interaction.” The articles appearing in these pages were contributed by speakers at the colloquium Any opinions, findings, conclusions, or recommendations expressed in this volume are those of the authors and do not necessarily reflect the views of the National Academy of Sciences The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters Dr Bruce M Alberts is president of the National Academy of Sciences

Library of Congress Cataloging-in-Publication Data Chemical ecology : the chemistry of biotic interaction / Thomas Eisner and Jerrold Meinwald, editors

cm

Includes bibliographical references and index

ISBN 0-309-05281-5 (alk paper)

1 Chemical ecology I Eisner, Thomas, 1929-— II Meinwald, Jerrold, 1927—

Printed in the United States of America

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Table of Contents

Preface Thomas Eisner and Jerrold Meinwald

The Chemistry of Defense: Theory and Practice May R Berenbaum

The Chemistry of Poisons in Amphibian Skin John W Daly

The Chemistry of Phyletic Dominance Jerrold Meinwald and Thornas Eisner

The Chemistry of Social Regulation:

Multicomponent Signals in Ant Societies Bert Holldobler

The Chemistry of Eavesdropping, Alarm, and Deceit Mark K Stowe, Ted C J Turlings, John H Loughrin,

W Joe Lewis, and James H Tumlinson

Polydnavirus-Facilitated Endoparasite Protection Against Host Immune Defenses

Max D Summers and Sulayman D, Dib-Hajj

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Chemical Ecology: The Chemistry of Biotic Interaction (1998)

iv / Contents

The Chemistry of Gamete Attraction:

Chemical Structures, Biosynthesis, and (A)biotic Degradation of Algal Pheromones Wilhelm Boland

The Chemistry of Sex Attraction Wendell L Roelofs

The Chemistry of Sexual Selection Thomas Eisner and Jerrold Meinwald

The Chemistry of Signal Transduction Jon Clardy

Chemical Signals in the Marine Environment:

Dispersal, Detection, and Temporal Signal Analysis Jelle Atema

Analysis of Chemical Signals by Nervous Systems John G Hildebrand

Chemical Ecology:

A View from the Pharmaceutical Industry Lynn Helena Caporale

List of Abbreviations Index

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Preface

urs is a world of sights and sounds We live by our eyes and ears and tend generally to be oblivious to the chemical happenings in our surrounds Such happenings are ubiquitous All organisms engender chemical signals, and all, in their respective ways, respond to the chemical emissions of others The result is a vast communicative interplay, fundamental to the fabric of life Organisms use chemicals to lure their mates, associate with symbionts, deter enemies, and fend off pathogens Chemical ecology is the discipline that is opening our “eyes” to these interactions It is a multifaceted discipline, intent on deciphering both the chemical structure and the information content of the mediating molecules And it is a discipline in which discovery is still very much in order, for the interactions themselves remain in large measure to be uncovered Chemical ecology has made major progress in recent decades This reflects, in part, the extraordinary technical innovation that has taken place in analytical chemistry Highly improved procedures are now available for separating complex mixtures into their individual components, as well as for quantitating and chemically characterizing desig- nated compounds There has also been a vast increase in the sensitivity of the techniques Where gram quantities were once needed for elucidation

of chemical structure, milligram or even microgram quantities may now suffice These refinements in sensitivity are of particular importance, given that organisms often produce their signal molecules in vanishingly small amounts

Progress in chemical ecology has also been fostered by advances in biology itself Chemical interactions in nature are often social, in the sense

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vi / Thomas Eisner and Jerrold Meinwald that they occur between conspecifics Conceptual advances in behavioral biology, particularly sociobiology, have helped put new slants on inquiries into such social phenomena as mate attraction, sexual selection, parental investment, caste determination, and colony organization, all frequently mediated by chemicals The questions themselves, answered

at one level of organization, often lead to inquiries at another level Studies of pheromones, for instance, first with insects and then with selected mammals, were doubtless influential in prompting the highly promising current inquiries into pheromonal communication in humans Other biological disciplines are also proving relevant Virtually every chemically mediated interspecific interaction, whether between predator and prey, herbivore and plant, or parasite and host, lends itself to interpretation in the broadest evolutionary, ecological, population- biological, and molecular-biological terms

Molecular biology may, in fact, increasingly shape the questions that are asked in chemical ecology How do given signal molecules arise in the course of evolution? How are they synthesized, and how is the rate and timing of their production controlled? How are they recognized at the level of the receptor? How do noxious chemical signals, designed to repel

or poison, affect their intended targets? How is it that receiver organisms are sometimes able to circumvent, counteract, or even secondarily em- ploy, such offensive chemicals? Molecules that transmit information between organisms are a fundamental part of the regulatory chemicals of nature The rules that apply to intraorganismal chemical regulation apply in large measure to them as well

Molecules that have signal value in nature sometimes prove to be of use to humans One need only cite the example of medicinals to underscore the point Major recent additions to our therapeutic arsenal include ivermectin, cyclosporin, FK-506, and taxol, compounds that can all be expected to have evolved as signaling agents Many and varied benefits can be expected to be derived from an ongoing search for natural products Chemical ecologists should become active participants in this search They have the expertise, gained through laboratory and field experimentation and observation, to rate species by “chemical promise” and therefore to aid in the important task of selecting species for chemical screening Chemical ecologists are also in a position to provide some assessment of the hidden value of nature The search for natural products has essentially only begun Most species, especially microbial forms and invertebrates, remain to be discovered, let alone to be screened for chemicals What remains unknown is of immense potential value, and deserving of protection, lest we be forever impoverished by its loss To help in the preservation effort, chemical ecologists will need to speak out

as conservationists

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Preface / vii The essays that follow are synopses of lectures delivered at a colloquium on chemical ecology Almost 150 participants attended the pro- ceedings The papers do not provide an overview of the discipline but rather give a glimpse into selected research areas that are contributing to advancement of the field We are immensely grateful to our invited speakers, both for the quality of theix communications and for the personal enthusiasm they brought to the meeting Discussions were convivial and much enlivened by the youthfulness of most of the audience Four participants, Jan T Baldwin, Gunnar Bergstrém, Arnold Brossi, and Amos B Smith IJ deserve special thanks, for presiding over the sessions and for leading the discussions We are also grateful to Jack Halpern, Vice President of the Academy, for asking us to organize the colloquium, and to Bruce Alberts, President of the Academy, for provid- ing introductory remarks at the meeting For help in preparation of the colloquium we are indebted to Kenneth R Fulton and Jean Marterre of the Academy and especially to our Cornell associates, Janis Strope and Johane Gervais

Financial resources for the colloquium were provided by the Academy and supplemented by contributions from a number of industries (American Cyana- mid Company, E I DuPont de Nemours & Co., Givaudan—Roure Corporation, Merck & Co., Monsanto, Rohm and Haas Company, Schering-Plough Research Institute, Sterling-Winthrop Inc., Syntex, Takasago International Corporation, Zeneca Inc.), to which we are much indebted

Thomas Eisner and Jerrold Meinwald

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The Chemistry of Defense:

Theory and Practice

MAY R BERENBAUM

interspecific interactions are rarely the same substances used by an organism to meet the daily challenges of living, such as respiration, digestion, excretion, or, in the case of plants, photosynthesis They are, in both plants and animals, of ‘a more secondary character’ [to borrow a phrase from Czapek (1)] These secondary compounds are generally derived from metabolites that do participate in primary physiological processes In plants, for example, secondary compounds such as alkaloids, coumarins, cyanogenic glycosides, and glucosinolates derive from amino acids; tricarboxylic acid cycle constituents are involved in the formation of polyacetylenes and polyphenols; glucose, aliphatic acids and other “primordial molecules” (2) play a role not only in primary metabolism but in secondary metabolism as well In insects, many defensive secretions are derived from the same amino acids used to construct proteins [among them, quinones in many beetles and cock- roaches derive from tyrosine, formic acid in ants from serine, isobutyric acid in swallowtail caterpillars from isoleucine and valine, and alkyl sulfides in ants from methionine (3, 4)] Presumably, secondary compounds are physiologically active in nonconspecific organisms precisely because of their secondary nature; it is to be expected that most organisms possess effective means for metabolizing, shunting around, or

May Berenbaum is professor and head of the Department of Entomology at the University of Illinois, Urbana-Champaign.

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otherwise processing primary metabolites and it is the unusual chemical that circumvents these mechanisms to cause toxicity

Unlike primary metabolites, which are practically universal constituents of cells, tissues, and organs, secondary compounds are generally idiosyncratic in distribution, both taxonomically and ontogenetically Chlorophyll, for example, the principal photosynthetic pigment, is found

in virtually all species of angiosperms, in virtually all life stages of virtually

all individuals In contrast, the furanocoumarins are secondary com-

pounds known from only a handful of angiosperm families (5) Within a species (e.g., Pastinaca sativa), there is variability in furanocoumarin content and composition among populations (6, 7); within an individual, there is variation among body parts during any particular life stage (8) and temporal variation in the appearance of these compounds over the course of development (9); there are even differences in the content of individual seeds, depending upon their location in an umbel (10), fertilization history (11), and their position within the schizocarp (12) Secondary chemicals are by definition taxonomically restricted in distribution, yet despite this fact there are patterns in production and allocation that transcend taxa (13) Their presence in an organism is generally characterized by specialized synthesis, transport, or storage Levels of abundance are subject to environmental or developmental regulation and, unlike primary constituents, which may be present in virtually all cells of an organism, chemical defenses are typically com- partmentalized, even in those cases in which the chemicals are acquired exogenously, as when sequestered from a food source There often exists

a system for external discharge, delivery, or activation, not only as a means of ensuring contact with a potential consumer but also as a means

of avoiding autotoxicity until a confrontation arises; and of course these compounds are almost invariably, by virtue of structure, chemically reactive (e.g., able to be taken up by a living system, to interact with a receptor or molecular target, and to effect a change in the structure of the molecular target) The remarkable convergence of structural types in plant and insect secondary metabolites is at least suggestive that the processes leading to biological activity in both groups share certain fundamental similarities (14)

Secondary chemicals can be said to be defensive in function only if they protect their producers from the life-threatening activities of another organism Distinguishing between offensive and defensive use of chemicals is difficult, and present terminology does little to assist in making that distinction The term “allomone’”’ is frequently used synonymously with “chemical defense,” yet allomones are not necessarily defensive in function An allomone has been defined as a chemical substance beneficial to its producer and detrimental to its recipient (15), so chemicals used

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The Chemistry of Defense / 3

by a predator to lure prey (16) are rightly regarded as allomonal but are not obviously defensive By the same token, chemicals that reduce compe- tition for limited resources, clearly beneficial to the producer, may be defensive of those resources but are not necessarily defensive in the life of the organism producing them Allelopathic compounds produced by a plant species may increase fitness of that plant by preempting a resource, such

as water or soil nitrogen, that might otherwise be exploited by other plants (17), but in the sense that such compounds can kill potential competitors (such as nonconspecific seedlings) they are used in an offensive fashion, as for range expansion at the expense of another organism

A defensive chemical, then, is a substance produced in order to reduce

the risk of bodily harm As such, most are poisons—defined as “any agent which, introduced (especially in small amount) into an organism, may chemically produce an injurious or deadly effect’ (18) This rather restrictive definition may not be universally embraced by chemical ecologists On one hand, the definition implies an interaction with another organism and, particularly with respect to plants, secondary compounds may fulfill many functions in the life of the producer organism other than producing injurious or deadly effects on other organisms (19, 20) Many plant secondary compounds, for example, are inducible by UV light and presumably serve to protect (or “defend”) plants from damaging effects of UV exposure (21); by no stretch of the imagination can such compounds be considered poisons, since they exert

no injurious effects on the damaging agent, the sun In this context, they can no more be considered ‘‘defenses’’ than cell wall constituents can be considered “defenses” against gravity On the other hand, some investigators, while acknowledging the fact that secondary chemicals have deleterious effects on other organisms, are reluctant to ascribe their presence, particularly in plants, to selection pressure exerted by those organisms (22-24) Calling certain secondary chemicals “defenses would be giving credence to the assertion that they exist only by virtue of

the selection pressures exerted by consumers Nonetheless, an examuna-

tion of the distribution, pattern of allocation, chemical structure, and modes of action of secondary compounds in a broad cross section of organisms reveals so many striking convergences and similarities that the notion that variation in the distribution and abundance of chemicals that act as poisons results at least in part from selection by consumer organisms certainly seems tenable, if not inescapable

DISTRIBUTION OF DEFENSES One line of evidence, admittedly circumstantial, that consumers have influenced the evolution of chemical defenses is their taxonomic distri-

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4/ May R Berenbaum

bution There are entire phyla in which chemical defenses have never been identified (Table 1) Undoubtedly, in many cases this absence of chemical defenses may result simply from an absence of studies explicitly designed to discover them—for many small, obscure organisms, life histories, let alone chemistries, are poorly known This problem may not

be as severe as it might appear, because chemically defended organisms often call attention to themselves by way of aposematic coloration (Table 1) (in fact, it may well be that effective defenses, particularly chemical ones, may be a prerequisite for a conspicuous life-style among smaller organisms) Nonetheless, any reported absence of chemical defense may be artifactual due to incomplete information With that caveat in mind, it is interesting to note that conspicuously abundant on the list of the chemically defenseless are phyla comprised exclusively of parasitic animals As well, chemical defenses are absent in entirely parasitic orders within classes (Phthiraptera and Siphonaptera in the class Insecta, for example) These organisms are subject to mortality almost exclusively by their hosts, and poisoning or otherwise severely impairing

a host is unlikely to enhance lifetime fitness of a parasite (particularly those parasites that cannot survive more than a few hours without one)

Chemical defenses are also rare in organisms at the top of the food chain—-organisms that are themselves at low risk of being consumed Large vertebrates, by virtue of size, speed, and strength, often occupy that position in both terrestrial and aquatic ecosystems (carnivores and odontocetes, for example), Chemically defended mammals include skunks and the duck-billed platypus, both opportunistic scavengers (32) Among birds, chemical defense has been demonstrated to date only in the pitohui (25), which feeds on leaf litter invertebrates (J Daly; ref 79), but likely exists in the conspicuously colored female hoopoe, which “has a strongly repulsive musty smell that emanates from her preen gland, and

is believed to have a protective function like attar of skunk’ (33) Hoopoes are also opportunistic feeders that consume debris along with insects and other invertebrates It is somewhat surprising that chemical defenses are not more frequently encountered among small birds, but the absence of reports may be due to the tendency of investigators to assume conspicuous plumage results from sexual selection, rather than aposema- tism and distastefulness (25)

In contrast with fast, strong predators, organisms with a limited range

of movement, or limited control over their movements—those that cannot run away from potential predators—are well represented among the chemically defended (Table 1) Sessile marine invertebrates are particularly accomplished chemists; these include in their ranks sponges, antho- zoan corals, crinoid echinoderms, polychaetes, bryozoans, brachiopods,

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The Chemistry of Defense / 5 TABLE 1 General survey of the distribution of chemical defenses (3, 25-30) and certain life history characteristics (31) in animals

Animal phyla in which chemical defenses are rare or unreported Platyhelminthes,*t 15,000 spp Phoronida,t 10 spp Chordata: Vertebrata Rhynchocoela,t 650 spp Pogonophora,t 80 spp Chondrichthyes, 800 spp Gnathostomulida, 80 spp Onychophora, 70 spp Aves,t 8600 spp Gastrotricha, 400 spp Echiurida, 100 spp Mammalia, 4500 spp Rotifera, 2000 spp Tardigrada, 400 spp

Kinorhyncha, 100 spp Pentastomida,* 60 spp

Acanthocephala,* 500 spp Priapulida, 9 spp

Nematoda,” 1500 spp Chaetognatha, 65 spp

Nematomorpha,* 100 spp Chordata: Cephalochordata, 28 spp

Animal phyla in which autogenous chemical defenses are documented Porifera,tt 5000 spp

Sesquiterpenes, sesterterpenes, dibromotyrosine derivatives, isonitriles, isothiocyanates, polyaikylated indoles, macrolides, quinones, ancepsenolides, sterols

Coelenterata:tt Anthozoa Aleyonaria, 9000 spp

Sesquiterpenes, diterpenes, alkaloids, prostaglandins, pyridines

Peptides, proteins Ectoprocta,t

Tambjamine pyrroles, 4000 spp

Brachiopoda,† 300 spp

Mollusca:† Gastropoda, 50,000 spp

Opistobranchia Sesquiterpene dialdehydes, dimenoic acid glycosides, haloethers Prosobranchia

‘Triterpenes Pulmonata Polyproprionates Annelida (Polychaetat}), 5300 spp

Phenolics Arthropoda:t 800,000 spp

Insecta Hydrocarbons, alcohols, aldehydes, ketones, carboxylic acids, quinones, esters, lactones, phenolics, steroids, alkaloids, cyanogenic glycosides, sulfides, peptides, proteins Arachnida

Quinones, alkaloids, cyanogenic glycosides Echinodermata,t 6000 spp

Holothuroidea Steroidal glycosides, saponins Crinozoa†

Polyketide sulfates Asterozoa

Phenolics, saponins, steroidal glycosides Chordata, 1250 spp

Tunicatatt Bipyrrole alkaloids, cyclic peptides, quinones, macrolides, polyethers Vertebrata: Osteichthyest, 22,000 spp

Alkaloids, peptides Amphibia,t 3150 spp

Alkaloids Reptilia,t 7000 spp

Alkaloids, hydrocarbons, aldehydes, acids

*Many species parasitic

tMany species conspicuously colored

tMany species sessile.

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6 / May R Berenbaum and tunicates (26, 34) Completely consistent with the pattern is the

virtually universal presence of toxins in plants, ranging from mosses to angiosperms (4), most of which remain firmly rooted to the ground for most of their lives and occupy the bottom rung of most food chains It is interesting to note that chemically defended taxa tend to be more speciose than those lacking chemical defenses, but whether this relationship reflects sampling vagaries or causation is anybody’s guess

PATTERNS OF ALLOCATION Secondary chemistry differs from primary chemistry principally in its distributional variability and it is this variability that has intrigued ecologists for the past 30 years Theories [or provisional hypotheses (35)]

to account for the structural differentiation and function of secondary metabolites, as well as the differential allocation of energy and materials

to defensive chemistry, abound, but they are almost exclusively derived from studies of plant-herbivore interactions (Table 2) This emphasis may be because the function of secondary chemicals in plants is less immediately apparent to humans, who have historically consumed a broad array of plants without ill effects, so alternative explanations of their presence readily come to mind The fact that animals upon distur- bance often squirt, dribble, spray, or otherwise release noxious substances at humans and cause pain leads to readier acceptance of a defensive function [although there are skeptics who are unconvinced of a

TABLE 2 Chemical defense theories

Ref Theories to account for allocation of chemical defenses in plants

Latitudinal pest pressure gradients 37

Theories to account for allocation of chemical defenses in animals

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The Chemistry of Defense / 7 defensive function of certain animal secondary compounds—Portier (48), for example, reports that ‘‘Certains auteurs voient dans les glandes nucales (of swallowtail caterpillars) un appareil d’élimination de substances toxiques ou tout au moins inutilisables contenus dans la nourri-

occupy a unique place in theories of chemical defense allocation is unclear Plants produce secondary compounds as derivatives of primary metabolism; animals do the same In fact, plants may be rather unrepre- sentative of chemical defense strategies as a whole in that they rarely coopt defense compounds from other organisms via sequestration, although there are exceptions to the general rule [e.g., parasitic plants (49-51)]

The relative importance of consumer selection pressure in determining patterns of production of secondary compounds varies with the theory Coley et al (44) suggest that resource availability and the concomitant growth rate of a plant, more than its potential risk of herbivory or its historical association with herbivores, determine the type and quantity of chemical defenses in plants; while “the predictability of a plant in time and space may influence the degree of herbivore pressure it should be included as a complementary factor,” rather than as the sole driving force

in the evolution of chemical defenses and their allocation patterns Bryant

et al (43) suggest that carbon and nutrient availability alone can determine patterns of chemical defense allocation; according to this hypothesis, “environmental variations that cause changes in plant carbohydrate status will lead to parallel changes in levels of carbon-based secondary metabolites” (52) Such theories, along with the contention that “the evolution of plant defense may have proceeded independent of consumer adaptation’ (23), are in many ways reincarnations of ‘‘overflow metabolism” or “biosynthetic prodigality” hypotheses that reappear intermittently (53, 54) Yet how overflow metabolism can generate and maintain biochemical diversity—hundreds of biosynthetically distinct and unique classes of secondary metabolites—is an enigma

Essentially (and rightly) undisputed is the fact that novel secondary compounds arise by genetic accident—by mutation or recombination—

so it is not altogether surprising that, given the idiosyncratic nature of mutations, the distribution of biosynthetic classes of compounds is idiosyncratic as well At issue, however, is how certain mutations become established within a population or species Mutant individuals can increase In representation in populations either as a result of positive selection or as a result of random genetic events, such as drift; fixation by drift occurs only when there is no negative selection against the trait It is

a virtual certainty that at least some portion of the chemical variability of plants (and probably of all organisms) is nonadaptive—vestiges of past

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8 / May R Berenbaum selection pressures no longer experienced due to extinctions, transient occurrences of secondary metabolites generated by indiscriminant enzymatic transformations, and the like (23) But predictable and highly specific accumulation of particular types of chemicals in taxonomically related species in particular organs in particular portions of the life cycle regulated by promoters that respond to chemical cues from consumer organisms (55, 56) seems inconsistent with such nonspecific processes Very little discussion to date in plant—insect interactions has centered

on why certain secondary metabolites are built the way they are and why they act the way they do on consumer organisms In vertebrates,

“overflow metabolism” ends up almost exclusively converted to adipose tissue, despite the demonstrable ability of at least a few vertebrates to manufacture secondary metabolites; such tissue, in times of nutrient deprivation, is in fact readily mobilized by its producer Why plants, which have the capacity to make glucose and, from glucose, the storage material starch, should make secondary compounds as “overflow” metabolites is unclear Since glucose is a starting material for much of secondary metabolism, it is difficult to conceive of how such elaborate pathways could evolve in the absence of any selection pressure other than whatever problems may be associated with fixing too many carbon atoms The suggestion that “high tissue carbohydrate carbon concentra- tions cause more carbon to flow through pathways leading to the synthesis of carbon-based secondary metabolites [and that] this effect of mass action on reaction rates is stronger than any enzymatic effects such as feedback inhibition due to end-product accumulation” (52) runs counter to the many observations that secondary metabolite production is otherwise finely regulated by enzymes physically, temporally, and de- velopmentally within a plant (57) In fact, such physiological responses to sunlight sound positively pathological

Recent allocation theories generally classify chemicals based on criteria other than specific structure or biosynthetic origin “Carbon-based” compounds (43, 52) are considered as a more-or-less homogeneous group, despite the fact that they include biosynthetically unrelated groups with wildly different activities as well as transport and storage requirements The same is true for “IN-based”” compounds; the dichotomy between N-based and C-based compounds does not appear to take into consideration the fact that N-based compounds may actually contain more carbon atoms than smaller C-based compounds, and C-based compounds may require more investment in N-based enzymes for synthesis and storage than do many N-based compounds Rather, there is

a focus on whether or not secondary compounds are easily metabolized,

or turned over, by the plants producing them [as in Coley et al (44),

“mobile” and ‘immobile’ defenses] or whether they are accumulated in

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The Chemistry of Defense / 9 large or small quantities (39) Yet more defense does not necessarily lead

to better defense—adding small amounts of biosynthetically different chemicals may by synergism enhance existing defenses more effectively than greatly increasing the concentration of those existing chemicals (58) Such interactions cannot be evaluated if only a single type of chemical is quantified Secondary chemicals are not like muscles—pumping them up

is not always the most effective strategy for overcoming an opponent Despite the growing number of studies failing to confirm at least one of the predictions of the carbon/nutrient balance hypothesis [24 such studies are cited in Herms and Mattson (59)], it remains popular as a testable hypothesis, as do several of its predecessors as well as its successors In fact, none of these theories has really ever been resound- ingly rejected; they all more-or-less coexist, by virtue of supportive evidence in some system or other Studies of plant—herbivore interactions are in a sense unique in the field of chemical ecology; no other area is quite so rife with theory One problem with attempting to develop an all-encompassing theory to account for patterns of defense allocations [as called for by Price (35) and Stamp (60)] is that such a theory may not be

a biologically realistic expectation even for just the plant kingdom Most theories certainly suit the specific systems from which they were drawn, which of necessity constitute only a tiny fraction of all possible types of interactions; it is when they are generalized that the fit breaks down That many theories coexist is at least in part due to the fact that they are not mutually exclusive—they all share certain elements If there is a recurrent theme in the past century of discussion, it is that chemical defenses confer

a benefit and exact a cost Much current controversy centers not on the existence of costs and benefits but on the magnitude of those values Resource availability hypotheses (43, 44) focus on material costs of production; herbivory-based hypotheses (39-42) focus on benefits ac- crued So if a ratio is to be tested, better that it simply be the benefit/cost ratio—that is, the benefits of a chemical, in terms of fitness enhancement

in the presence of consumers, relative to the costs of a chemical, in terms

of fitness decrements resulting from its production, transport, storage, or deployment

This approach is not without its shortcomings, the greatest of which is that costs and benefits have proved to be exceedingly difficult to measure There is far from a consensus on what constitutes an appropriate demonstration of costs of chemical defense (57, 61, 62) In many theoret- ical discussions of costs of defense, particularly in plants, costs are measured in terms of growth rates (44, 59, 63), rather than in terms of reproductive success In many empirical estimates of costs, the chemical nature of the defense is not defined (64) or secondary metabolites are measured in bulk (65), without any regard to their individual activities

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10 / May R Berenbaum

On the other side of the coin, measuring the benefits of synthesizing a particular chemical compound requires an intimate knowledge of the interactions of the producer organism with its biotic environment Bioassays of both plant and animal defenses tend to be done with isolated compounds against laboratory species that are easily reared (66); in the future, bioassays may need to be done with more ecologically appropriate species (and possibly with a whole suite of agents acting simulta- neously) (67) None of these requirements is likely to make this enterprise any more tractable than it is at the moment

There are advantages, however, in taking such a basic approach to understanding chemical defense allocation First of all, expressing costs and benefits in terms of fitness couches the discussion in an evolutionary

framework, a framework that is missing from many current discussions

of plant—-herbivore interactions No discussion of adaptation can be purely ecological, since the process of adaptation is an evolutionary one;

in all of the discussions of life history syndromes associated with defensive strategies (39-42, 59), virtually no evidence exists that any of the traits characterizing these syndromes are genetically based or, equally important, genetically correlated and likely to evolve in concert Current patterns of allocation observed today are the result of an evolutionary process and are likely to change in the future as a result of evolutionary processes; it is difficult to appreciate ecological patterns without at least a rudimentary understanding of their evolutionary underpinnings and understanding the evolutionary process necessitates identifying selective agents and quantifying the selective forces they exert

Restoring evolution to a place of prominence in future discussions of chemical defense means greatly increasing attention to the genetics of chemical defense production and allocation Research pursuits in the study of chemical defense that should receive a renewed interest in this context include (i) investigation of multiple classes of secondary metabolites within a species, (ii) establishment of the genetic basis for chemical variability within a species, (iii) testing for toxicological interactions and genetic correlations, (iv) determination of genetic correlations between chemistry and life history traits, (v) precise estimation of consumer effects

on fitness and influence of chemical variation on those effects, and (vi) elucidation of modes of action and mechanisms of genetically based counteradaptation in consumers All of these pursuits require a sophis- ticated understanding of the biochemistry of a particular system, such that individual metabolites, not functional categories of secondary compounds, are identified, quantified, and monitored The fact that secondary compounds may be involved in functions other than defense against consumers cannot be overlooked, particularly if expected genetic correlations fail to materialize

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The Chemistry of Defense /11

An additional advantage of examining chemical defense allocation in the simplified context of benefit/cost analysis in particular systems is that such an approach is essentially unbiased relative to taxon Theories derived from plant—herbivore interactions place an undue emphasis on the ability of plants to synthesize their primary metabolites from inorganic elements, but that ability may not necessarily be reflective, or predictive, of abilities to synthesize secondary metabolites There is undeniably some degree of linkage between primary and secondary metabolism—at the very least, dead plants do not manufacture secondary metabolites—but there is little evidence to support the linear relationship that is assumed to prevail between them If secondary metabolism cannot

be totally divorced from primary metabolism in studying the ecology and evolution of chemical defense, irrespective of whether it takes place in plants, animals, or any other organisms, it may be productive for a while

at least to arrange for a trial separation and see whether paradigms change

SPECIAL CASE: HUMAN CHEMICAL DEFENSES One compelling reason for examining patterns of chemical defense across taxa, and across trophic tiers, in order to distill out basic elements

is that such an approach is essential in understanding chemical defense allocation in a seriously understudied species—Homo sapiens By all rights and purposes, human beings should not utilize chemical defenses; as top carnivores in many food webs, they are rarely if ever consumed by other organisms [but see ‘‘Little Red Riding Hood,” in Lang (68)] Notwith- standing, humans have used every variant on chemical defense mani- fested by other organisms (Table 3) Like insects (69), parasitic plants (49-51), some birds (70), marine invertebrates (26, 67) and vertebrates (71), humans have coopted plant toxins to protect themselves against their consumers; the use of botanical preparations to kill insects, parasitic and otherwise, antedates written history (72) Many of the chemicals in use today as biocides, including antibiotics, are derived from other organisms Natural products provide +25% of all drugs in use today (73) and botanical insecticides (or their chemically optimized derivatives) are still widely used as pest control agents (74) More recently, like many species of plants (4), insects (3), marine invertebrates (26), and vertebrates (27), humans have taken to synthesizing their own defensive compounds from inorganic materials, primary metabolites or from small precursor molecules, although this synthesis takes place in a laboratory or factory rather than in a cell or gland In doing so, humans face many of the same challenges faced by other organisms that synthesize chemicals (Table 3)—in addition to the actual synthesis, the storage, transport, and

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12 / May R Berenbaum TABLE 3 Human chemical defense characteristics

Parallels between humans and other organisms

Material and energy construction costs Material and energy construction costs Shipping, storage, and handling costs Solubility, transportability, autotoxicity costs Delivery costs, environmental impact, Problems associated with delivery-—toxicity

and autotoxicity considerations to mutalists, increased visibility to

acquisition

Differences between humans and other organisms

In humans, chemicals are used In other organisms, chemicals are used

In a broadcast fashion In a tissue- or otherwise site-specific fashion

As single purified toxins As variable mixtures

avoidance of autotoxicity all exact an economic cost, and the product is

“selected” by consumers based on its efficacy and its range of uses relative to other products

Although there are similarities between humans and other organisms

in the acquisition of chemical defenses, there are striking differences in the deployment of these defenses (Table 3) Whereas most organisms use chemical defenses to minimize their own risk of being consumed, humans use chemicals in an offensive fashion, with the express purpose

of killing off not only potential consumer organisms but also potential competitors for food or shelter Throughout history, humans have even used chemicals to kill off conspecific competitors, a use of chemicals that

is certainly unusual in the natural world (75) But humans have for the most part adopted the chemicals used by other organisms as defenses without at the same time investigating the manner in which these chemicals are deployed

In general, chemical defenses in other organisms reflect the probability

of attack and relative risk of damage (42); humans have in recent years developed a tendency to use chemical defenses prophylactically, even when potential enemies are absent Humans tend to select those chemical agents that kill, rather than repel or misdirect, a large proportion of the target population; from a plant’s perspective, the ultimate goal of chemical defense is to avoid being eaten, a goal that is as achievable if the plant is never consumed in the first place as it is if the consumer dies after

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The Chemistry of Defense / 13 ingesting a mouthful of toxin-laced tissue Most organisms manufacture complex mixtures of chemicals for defense, some of which may actually

be inactive as pure compounds (58); humans tend to prefer highly active individual components It is perhaps the ecologically inappropriate deployment of these natural products (and their synthetic derivatives) by humans that has led to the widespread acquisition of resistance in all manner of target species and the concomitant loss of efficacy of these chemicals (76)

Efforts in recent years to identify natural sources of insecticidal and other pesticidal materials have increased, at least in part due to continu- ing problems with nontarget effects of synthetics and the widespread appearance of resistance to these compounds Natural products are thought to offer greater biodegradability and possibly greater specificity than synthetic organic alternatives (77) However, the search for new biocidal agents is being conducted essentially as it has been done for the past century, with mass screening, isolation of active components, and development of syntheses for mass production of the active components (77, 78) Little or no effort is being expended by those interested in developing new chemical control agents in elucidating the manner in which plants or other source organisms manufacture, store, activate, transport, or allocate these chemicals By understanding the rules according to which organisms defend themselves chemically, there is perhaps as much to be gained, in terms of developing novel and environmentally stable approaches to chemical control of insects and other pests, as there

is by isolating and identifying the chemical defenses themselves

SUMMARY Defensive chemicals used by organisms for protection against potential consumers are generally products of secondary metabolism Such chemicals are characteristic of free-living organisms with a limited range of movement or limited control over their movements Despite the fact that chemical defense is widespread among animals as well as plants, the vast majority of theories advanced to account for patterns of allocation of energy and materials to defensive chemistry derive exclusively from studies of plant-herbivore interactions Many such theories place an undue emphasis on primary physiological processes that are unique to plants (e.g., photosynthesis), rendering such theories limited in their utility or predictive power The general failure of any single all-encompassing theory to gain acceptance to date may indicate that such a theory might not be a biologically realistic expectation In lieu of refining theory, focusing attention on the genetic and biochemical mechanisms that underlie chemical defense allocation is likely to provide greater insights

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into understanding patterns across taxa In particular, generalizations derived from understanding such mechanisms in natural systems have immediate applications in altering patterns of human use of natural and synthetic chemicals for pest control

1 thank Thomas Eisner and Jerrold Meinwald for asking me to think in broad

terms about chemical defenses and for serving as an inspiration to me for the past

two decades, and I thank Arthur Zangerl and James Nitao for their comments,

insights, and unique good cheer This work was supported in part by National

Science Foundation Grant DEB 91-19612

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Secondary Plant Metabolites, eds Rosenthal, G & Berenbaum, M (Academic, New York), pp 371-413

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The Chemistry of Poisons

in Amphibian Skin

JOHN W DALY

well known from plants, where they presumably serve in chemical defense against herbivores Poisons can also serve as venoms, which are introduced into victims by coelenterates; molluscs; various arthropods, including insects, spiders, and scorpions; gila monsters; and snakes, by a bite or sting, or as toxins, such as those produced by bacteria, dinoflagellates, and other microorganisms Examples of poisons of plant origin encompass a wide range of substances, including many alkaloids;

a variety of terpenes and steroids, some of which occur as saponins; and unusual secondary metabolites such as the trichothecenes, pyrethroids, and dianthrones (1, 2) Another wide range of presumably defensive substances occur in marine invertebrates, including steroid and terpenoid sapogenins, tetrodotoxins, a variety of polyether toxins, and alkaloids (3, 4) Poisons also occur in terrestrial invertebrates and vertebrates, where they serve as chemical defenses by insects and other arthropods (5, 6), by fish (7), and by amphibians (8) Recently, a toxic alkaloid was characterized from the skin and feathers of a bird (9), where it confers some protection against predation by humans Chemical defenses can be directed either against predators or against microorganisms The present paper is concerned with the chemical nature, origin, and function of poisons present in amphibian skin Many of the substances in amphibians Pisce substances occur throughout nature and are particularly

John Daly is chief, Laboratory of Bioorganic Chemistry, at the National Institutes of Health, Bethesda, Maryland

17

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18 / John W Daly

might better be categorized as ‘noxious’ rather than “poisonous,’”’ although at high enough dosages all of these compounds would be poisons

Toads and salamanders have been considered noxious creatures for centuries and indeed the majority of amphibians have now been found to contain noxious and sometimes poisonous substances in their skin secretions (8) The type of biologically active substance found in amphibians appears to have phylogenetic significance Thus, indole alkylamines are typically present in high levels in bufonid toads of the genus Bufo, phenolic amines in leptodactylid frogs, vasoactive peptides in a great variety in hylid frogs, particularly of genus Phylomedusa (10), and bufadienolides in parotoid glands and skins of bufonid toads of the genus Bufo as well as in skin of related bufonid genera Atelopus and probably Dendrophryniscus and Melanophryniscus (11) The water-soluble alkaloid tetrodotoxin occurs in newts of the family Salamandridae, toads of the brachycephalid genus Brachycephalus and the bufonid genus Atelopus, and now in one frog species of the dendrobatid genus Colostethus (12) Lipophilic alkaloids have been found only in salamanders of the salaman- drid genus Salamandra; in frogs of the dendrobatid genera Phyllobates, Dendrobates, Epipedobates, and Minyobates, the mantellid genus Mantella and the myobatrachid genus Pseudophryne; and in toads of the bufonid genus Melanophryniscus More than 70 other genera from 11 amphibian families do not have skin alkaloids The distribution of various lipophilic alkaloids in amphibians is given in Table 1 and structures are shown in Figure 1

The origin and function of poisons and noxious substances found in amphibians are only partially known The high levels of amines, including such well-known biogenic amines as serotonin, histamine, and tyramine and derivatives thereof, found in skin of various toads and frogs (8), undoubtedly are synthesized by the amphibian itself They are stored

in granular skin glands for secretion upon attack by a predator, where- upon their well-known irritant properties on buccal tissue would serve well in chemical defense The high levels of vasoactive peptides, such as bradykinin, sauvagine, physaelaemin, caerulein, bombesin, dermorphins, etc., presumably also serve in defense against predators, although many, including the magainins, have high activity as antimicrobials (13) and thus might also serve as a chemical defense against microorganisms Skin secretions from one hylid frog are used in “hunting magic” folk rituals by Amazonian Indians; such secretions contain many vasoactive peptides (10) and a peptide, adenoregulin, that can affect central adenosine receptors (14) The peptides of frog skin are synthesized by the amphibian and indeed additional peptides are being deduced based on cDNAs for their precursors (15) The various hemolytic proteins of certain amphib-

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22 / John W Daly ians are certainly of endogenous origin The steroidal bufadienolides appear to be synthesized from cholesterol by the bufonid toads (16) It has been suggested that structurally similar and toxic lucibufagins of fireflies might also be produced by the insect from dietary cholesterol (17) However, toxic cardenolides in monarch butterflies appear to be sequestered from milkweed plants by the larvae (18) The chemical defensive attributes of the highly toxic bufadienolides are due to effects on membrane Na /K*-ATPase

The origin of tetrodotoxins in amphibians and higher organisms remains enigmatic Thus, puffer fish raised in hatcheries do not contain tetrodotoxin (19), and likely biosynthetic precursors are not incorporated into tetrodotoxin with newts (20) Feeding nontoxic puffer fish with tetrodotoxin does not result in sequestration, but feeding toxic ovaries from wild puffer fish does (19) A bacterial origin for tetrodotoxin has been suggested, but such a source fails to explain the fact that one Central American species of toad of the genus Atelopus contains mainly tetrodotoxin; another Central American species contains mainly chiriquitoxin,

which is a unique but structurally similar toxin; and yet another contains

mainly zetekitoxin, which is another unique, probably structurally related toxin (see ref 12) Chiriquitoxin, while related in structure to tetrodotoxin, differs in the carbon skeleton (21) The chemical defensive attributes of tetrodotoxin are due to blockade of voltage-dependent sodium channels and hence cessation of neuronal and muscle activity The origin of the lipophilic alkaloids in dendrobatid frogs, engendered

by the observation that the frogs, which are used by Colombian Indians

to poison blow darts, when raised in captivity contain none of the toxic batrachotoxins present in wild-caught frogs (22), remains to be investi- gated In contrast, the toxic samandarines from fire salamanders are present in the skin glands of the salamander through many generations of nurture in captivity (G Habermehl, personal communication) The various lipophilic alkaloids of amphibians all have marked activity on ion channels and hence through such effects would serve effectively as chemical defenses, even though some have relatively low toxicity The batrachotoxins were the first class of unique alkaloids to be characterized from skin extracts of frogs of the family Dendrobatidae (see ref, 23 for a review of amphibian alkaloids) Batrachotoxin was detected

in only five species of dendrobatid frogs and these frogs were then classified as the monophyletic genus Phyllobates, based in part on the presence of batrachotoxins (24) However, levels of batrachotoxins differ considerably, with the Colombian Phyllobates terribilis containing nearly 1

mg of batrachotoxins per frog, while the somewhat smaller Phyllobates bicolor and Phyllobates aurotaenia, also from the rain forests of the Pacific versant in Colombia, contain 10-fold lower skin levels (8) The two

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The Chemistry of Poisons in Amphibian Skin / 23 Phyllobates species from Panama and Costa Rica contain either only trace amounts of batrachotoxin or for certain populations of Phyllobates lugubris

no detectable amounts Batrachotoxins are unique steroidal alkaloids, which were unknown elsewhere in nature until the recent discovery of homobatrachotoxin at low levels in skin and feathers of a Papua New Guinean bird of the genus Pitohui (9) In the dendrobatid frogs, three major alkaloids are present—namely, batrachotoxin, homobatrachotoxin, and a much less toxic possible precursor, batrachotoxinin A The latter, when fed to nontoxic captive-raised P bicolor using dusted fruit flies, is accumulated into skin glands but is not converted to the more toxic esters batrachotoxin and homobatrachotoxin (25) Dendrobatid frogs of another genus would not eat the batrachotoxinin-dusted fruit flies Batrachotoxins depolarize nerve and muscle by specific opening of sodium channels; the sodium channels of the Phyllobates species are insensitive to the action of batrachotoxin (22)

Further examination of extracts of dendrobatid frogs over nearly 3 decades led to the characterization of nearly 300 alkaloids, representing some 18 structural classes (see ref 23) Several classes remain unknown in nature except in frog skin (see Table 1), and their origin remains obscure

in view of the relatively recent finding that frogs of the dendrobatid genera Dendrobates and Epipedobates, like Phyllobates, do not have skin alkaloids when raised in captivity (26) The distribution of the various alkaloids of amphibians is pertinent to any speculation as to their origin (see Table 1)

The so-called pumiliotoxin A class of ‘‘dendrobatid alkaloids” is as yet known only in nature from frog/toad skin The class consists of alkaloids with either an indolizidine (pumiliotoxins and allopumiliotoxins) or a quinolizidine (homopumiliotoxins) ring, in each case with a variable alkylidene side chain The pumiliotoxin A class occurs in skin of all of the amphibian genera that contain lipophilic alkaloids with the exception of the fire salamanders, which contain only samandarines In spite of a wide distribution in the alkaloid-containing frogs, there are species and/or populations of frogs that have no pumiliotoxin A class alkaloids or only trace amounts Members of pumiliotoxin A class are active toxins with effects on sodium and perhaps calcium channels and, thus, would serve well in defense against predators

Histrionicotoxins represent another major class of dendrobatid alkaloids They contain a unique spiropiperidine ring system and side chains with acetylenic, olefinic, and allenic groups Histrionicotoxins remain known in nature only from dendrobatid frogs of the general Phyllobates, Dendrobates, and Epipedobates They are probably absent in the tiny dendrobatid frogs of the genus Minyobates Histrionicotoxins were detected in a single Madagascan frog of the mantellid genus Mantella (27)

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24 / John W Daly obtained through the pet trade but have not been detected in any extracts

of several Mantella species collected in Madagascar (28) Histrionicotoxins

do not occur in all species of the above dendrobatid frog genera or in all populations of a single species (8) Their occurrence within populations of

a species on a single small island can vary from high levels to none The decahydroquinolines are the third major class of dendrobatid alkaloids still known only from frog/toad skin Decahydroquinolines occur in skin of all the frog/toad genera that have lipophilic alkaloids with the sole exception of the Australian myobatrachid frogs of the genus Pseudophryne that contain only (allo)pumiliotoxins and a series of indole alkaloids unique in nature to this genus of frogs—namely, the pseu- dophrynamines (29)

A series of simple bicyclic alkaloids could be considered to make up a major “izidine” class of alkaloids in the dendrobatid and other frogs These include the 3,5-disubstituted pyrrolizidines, the 3,5-disubstituted and 5,8-disubstituted indolizidines, and the 1,4-disubstituted quinolizidines The 3,5-disubstituted pyrrolizidines and the 3,5-disubstituted indolizidines are not unique to frogs, having been reported from ants (see ref 6) Ants thus represent a potential dietary source for such alkaloids in dendrobatid and other frogs Indeed, feeding experiments with ants of the genus Monomorium that contain a 3,5-disubstituted indolizidine and a 2,5-disubstituted pyrrolidine resulted in a remarkable selective accumulation, into the skin of the dendrobatid frog Dendrobates auratus, of the indolizidine but not of the pyrrolidine (25) It should be noted that some dendrobatid frogs do contain significant levels in skin of such 2,5- disubstituted pyrrolidines and of 2,6-disubstituted piperidines, neither of which appears to be sequestered into skin, at least by D auratus The 5,8-disubstituted indolizidines and 1,4-disubstituted quinolizidines remain as yet unknown in nature except from frog/toad skin (Table 1) There are also a number of alkaloids characterized from skin extracts of dendrobatid frogs that have a rather limited distribution within the many species that have been examined and are as yet known only from frog skin The tricyclic gephyrotoxins occur along with the more widely distributed histrionicotoxins in only a few species and populations of dendrobatid frogs (8) The tricyclic cyclopenta[b]quinolizidines occur in only one species, a tiny Colombian frog Minyobates bombetes (30) The potent nicotinic analgesic epibatidine occurs only in four dendrobatid species of the genus Epipedobates found in Ecuador (31)

Two classes of dendrobatid alkaloids have potential dietary sources The first are the pyrrolizidine oximes (32), whose carbon skeleton is identical to that of nitropolyzonamine, an alkaloid from a small millipede (33) Indeed, raising the dendrobatid frog D auratus in Panama on leaf-litter arthropods, gathered weekly, resulted in skin levels of the

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The Chemistry of Poisons in Amphibian Skin / 25 pyrrolizidine oxime 236 even higher than levels in wild-caught frogs from the leaf-litter site (34) The second are the tricyclic coccinelline alkaloids that have been found in several frogs/toads The coccinellines occur as defensive substances in a variety of small beetles (see ref 6) Thus, beetles represent a possible dietary source for coccinelline-class alkaloids in frog/toad skin Indeed, the beetle alkaloid precoccinelline is a significant alkaloid in the skin of D auratus raised in Panama on leaf-litter arthropods (34) The other alkaloids that were found in skin of D auratus raised

on leaf-litter arthropods are three other tricyclic alkaloids, perhaps of the coccinelline class but of unknown structure, two 1,4-disubstituted quinolizidines, a gephyrotoxin, a decahydroquinoline, and several histrionicotoxins With the exception of the pyrrolizidine oxime 236, skin levels of the various alkaloids in the captive-raised frogs were low compared to levels of alkaloids in wild-caught frogs from the leaf-litter collection site

or from the parental stock of D auratus on a nearby island (34) Individual variation in wild-caught frogs appears significant, which complicates the comparisons However, the lack of any pumiliotoxins and the relatively low levels or absence of decahydroquinolines and histrionicotoxins in the captive-raised frogs suggests that dietary sources for these alkaloids have been missed in the paradigm using large funnels to collect the arthropods from the leaf litter

In summary, poisons used in chemical defense are widespread in nature In amphibians, the defensive substances seem to be elaborated by the amphibian in the case of amines, peptides, proteins, bufadienolides, and the salamander alkaloids of the samandarine class For the tetrodotoxin class of water-soluble alkaloids, the origin is unclear, but symbiotic bacteria have been suggested for marine organisms (4) For the so-called dendrobatid alkaloids, a dietary source now appears a likely explanation for the lack of skin alkaloids in dendrobatid frogs raised in captivity Certainly, dendrobatid frogs of the dendrobatid genera Phyllobates, Den- drobates, and Epipedobates, which in the wild contain skin alkaloids, have highly efficient systems for accumulating selectively into skin a variety of dietary alkaloids (25, 34) A biological system for sequestration of alkaloids for chemical defense finds precedence in the transfer of pyrrolizidine alkaloids from plants via aphids to ladybug beetles (35) Accumu- Jation of cantharidins in muscle of ranid frogs after feeding on beetles has been documented (36) Frogs of the dendrobatid genus Colostethus, which

in the wild do not contain skin alkaloids, do not accumulate dietary alkaloids (25)

The proposal that all alkaloids found in skin glands of dendrobatid frogs and used in chemical defense against predators have a dietary origin leads to many questions First, the profile of alkaloids has been found in many instances to be characteristic of a species or a population

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26 / John W Daly Thus, either the systems responsible for sequestration of alkaloids differ

in selectivity among different species and/or populations of dendrobatid frogs or the small arthropod fauna presenting itself and used as a diet by different species and/or populations varies even within a small island The latter appears more likely It was noted that the dendrobatid frogs raised on leaf litter in Panama shared more alkaloids with a population of

D auratus from the leaf-litter site than they did with the parental population from a nearby island (34) The second major question con- cerns what small insects or other arthropods contain such toxic and/or unpalatable alkaloids as the batrachotoxins, the pumiliotoxins, and the histrionicotoxins, the decahydroquinolines, the 5,8-disubstituted indolizidines, the 1,4-disubstituted quinolizidines, and epibatidine It is remarkable that such small, presumably distasteful arthropods have escaped the attention of researchers Whether frogs intent on sequestering defensive alkaloids seek out such prey is unknown With regard to the frogs/toads from the Madagascan family Mantellidae, the Australian family Myoba- trachidae and the South American genus Melanophryniscus of the family Bufonidae, which also contain many of the dendrobatid alkaloids, it is unknown whether sequestering systems are present or even whether captive-raised frogs will lack skin alkaloids If such systems are present, then it is remarkable from an evolutionary standpoint that such unrelated lineages of toads/frogs have independently developed systems for sequestering alkaloids into skin glands from a diet of small, presumably noxious insects for use by the toad/frog in chemical defense

SUMMARY Poisons are common in nature, where they often serve the organism in chemical defense Such poisons either are produced de novo or are sequestered from dietary sources or symbiotic organisms Among vertebrates, amphibians are notable for the wide range of noxious agents that are contained in granular skin glands These compounds include amines, peptides, proteins, steroids, and both water-soluble and lipid-soluble alkaloids With the exception of the alkaloids, most seem to be produced

de novo by the amphibian The skin of amphibians contains many structural classes of alkaloids previously unknown in nature These include the batrachotoxins, which have recently been discovered to also occur in skin and feathers of a bird, the histrionicotoxins, the gephyrotoxins, the decahydroquinolines, the pumiliotoxins and homopumiliotoxins, epibatidine, and the samandarines Some amphibian skin alkaloids are clearly sequestered from the diet, which consists mainly of small arthropods These include pyrrolizidine and indolizidine alkaloids from ants, tricyclic coccinellines from beetles, and pyrrolizidine oximes, pre-

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The Chemistry of Poisons in Amphibian Skin / 27 sumably from millipedes The sources of other alkaloids in amphibian skin, including the batrachotoxins, the decahydroquinolines, the histrionicotoxins, the pumiliotoxins, and epibatidine, are unknown While it is possible that these are produced de nove or by symbiotic microorganisms,

it appears more likely that they are sequestered by the amphibians from

as yet unknown dietary sources

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The Chemistry of Phyletic Dominance

JERROLD MEINWALD AND THOMAS EISNER

hether we chose as our criterion Erwin’s generous estimate of

\ ` / 30 million species of insects (1) or the somewhat more modest

number favored by Wilson (2), it is clear that insects have achieved formidable diversity on Earth On dry land they literally reign supreme It has been estimated that there are some 200 million insects for each human alive (3) The eminence of the phylum Arthropoda among animals is very much a reflection of the success of the insects alone

A number of factors have contributed to the achievement of dominance

by insects They were the first small animals to colonize the land with full success, thereby gaining an advantage over latecomers Their exoskeleton shielded them from desiccation and set the stage for the evolution of limbs, tracheal tubes, and elaborate mouthparts, adaptations that were to enable insects to become agile, energetically efficient, and extraordinarily diverse in their feeding habits Metamorphosis opened the option for insects to exploit different niches during their immature and adult stages The evolution of wings facilitated their dispersal as adults, as well as their search for mates and oviposition sites And there were subtle factors such

as the acquisition of a penis (the aedeagus), which enabled insects to effect direct sperm transfer from male to female, without recourse to an outer aquatic environment for fertilization (3)

Jerrold Meinwald is Goldwin Smith Professor of Chemistry and Thomas Eisner is

Schurman Professor of Biology and director of the Cornell Institute for Research in Chemical Ecology at Cornell University, Ithaca, New York

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30 / Jerrold Meinwald and Thomas Eisner

A factor not often appreciated that also contributed to insect success is the extraordinary chemical versatility of these animals Insects produce chemicals for the most diverse purposes—venoms to kill prey, repellents and irritants to fend off enemies, and pheromones for sexual and other forms of communication The glands responsible for production of these substances are most often integumental, having been derived by special- ization of localized regions of the epidermis In insects, as in arthropods generally, the epidermis is fundamentally glandular, being responsible for secretion of the exoskeleton In their acquisition of special glands, arthropods appear to have capitalized upon the ease with which epider- mal cells can be reprogrammed evolutionarily for performance of novel secretory tasks The glandular capacities of arthropods are known to anyone who has collected these animals in the wild Arthropods com- monly have distinct odors and are often the source of visible effluents that they emit when disturbed (Figure 1) Much has been learned in recent years about the function and chemistry of arthropod secretions The insights gained have been fundamental to the emergence of the field of chemical ecology Our purpose here is to focus on some aspects of the secretory chemistry of these animals, with emphasis on a few recent discoveries

ARTHROPOD CHEMICAL DEFENSES

In our earliest collaborative publication, we described the dramatic chemical defense mechanism of the whip scorpion, Mastigoproctus gigan- teus (6) This ancient arachnid is able to spray a well-aimed stream of ~85% acetic acid [CH,;CO,H] containing 5% caprylic acid [CH;(CH,),CO,H] at

an assailant The role of the caprylic acid proved to be especially interesting: it facilitates transport of the acetic acid through the waxed epicuticle of an enemy arthropod It is likely that this simple strategy of using a lipophilic agent to enable a more potent compound to penetrate

a predator’s cuticle has helped this species to survive for as long as 400 million years Many other independently evolved arthropod defensive secretions make use of the same strategy (9) The whip scorpion defense mechanism helped us to appreciate the fact that chemistry need not be complex to be effective The virtue of simplicity is further illustrated by the defensive use of mandelonitrile and benzoyl cyanide, easily decom- posed precursors of hydrogen cyanide (HCN), by certain millipedes and centipedes (10, 11)

While arthropod defensive secretions often rely for their effect on well-known aliphatic acids, aldehydes, phenols, and quinones, there are many cases in which compounds capable of whetting the appetite of any natural products chemist are utilized For example, steroids play a

Tiêu đề	Chemical Ecology: The Chemistry of Biotic Interaction
Trường học	University of Natural Sciences
Chuyên ngành	Chemical Ecology
Thể loại	Thesis
Năm xuất bản	2023
Thành phố	Hanoi

Định dạng
Số trang	232
Dung lượng	8,94 MB